photoacid

A photoacid is a molecule whose acidity increases when it absorbs light, such that the pKa in the excited electronic state (pKa*) is lower than in the ground state. Upon photoexcitation, the molecule becomes a stronger Brønsted acid and can donate a proton to nearby bases, solvents, or framework sites. This acidic state is typically transient, because the molecule relaxes back to the ground state through fluorescence, phosphorescence, or internal conversion, restoring its original acidity.

Two broad mechanistic paths are common. In excited-state proton transfer (ESPT), the acid–base pair is formed within the same molecule or between the excited molecule and a hydrogen-bonded partner, leading to rapid proton transfer. Classic laboratory examples include pyranine (8-hydroxypyrene-1,3,6-trisulfonic acid, HPTS) and related naphthol sulfonates, which exhibit large shifts in acidity upon excitation and are used as fluorescence-based pH indicators. In other systems, photoacidity arises from photoinduced chemical changes that release protons, such as photoacid generators used in chemically amplified photoresists; these devices release strong acid upon exposure to light to catalyze polymerization reactions.

Applications of photoacids span photochemical catalysis, proton-coupled electron transfer studies, and the initiation of pH jumps in chemical and biological environments. They are also used as probes to study ultrafast proton transfer dynamics by time-resolved spectroscopy. Design strategies involve tuning electronic structure with substituents, controlling solvent and hydrogen-bond networks, and selecting chromophores with favorable excited-state lifetimes.

Although the concept has been known for several decades, ongoing research continues to expand the library of photoacids, improve understanding of pKa* values in various solvents, and extend uses in materials science and catalysis.